1. Field of the Invention
The present invention relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such characteristics of a patient. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
Among other blood constituents, physicians often desire to monitor levels of carbon dioxide in a patient's bloodstream. For example, decreased levels of carbon dioxide in the blood may be related to poor perfusion. Thus, assessment of carbon dioxide levels may be useful for diagnosing a variety of clinical states related to poor perfusion. One method of determining the level of blood carbon dioxide involves measuring carbon dioxide levels of respiratory gases. As carbon dioxide in the bloodstream equilibrates rapidly with carbon dioxide in the lungs, the partial pressure of the carbon dioxide in the lungs approaches the amount in the blood during each breath. Accordingly, physicians often monitor respiratory gases during breathing in order to estimate the carbon dioxide levels in the blood.
Respiratory gas analyzers typically function by passing electromagnetic radiation through a respiratory gas sample and measuring the absorption that is related to carbon dioxide. Often, the gas samples are collected with adapters that are fitted into patients being given respiratory assistance, such as patients under anesthesia or patients on life support systems, to connect between the endotracheal tube (ET tube) and the ventilating tube of the breathing apparatus. These tubes convey breathing gases to the patient and exhaled breath away from the patient. The airway adapter is in the form of a short connector of tubular shape, and the adapter is required to make a connection between the generally very different cross sections of these two tubes.
Respiratory gases may also be collected through the use of cannulas, which are flexible tubes that are threaded through the mouth or nose. Respiratory gas samples collected from a cannula may be aspirated from the airway stream and exposed to a carbon dioxide sensor.
It is often inconvenient to measure carbon dioxide in respiratory gases from respiratory gas samples collected from an intubation tube or cannula. Although these methods are considered to be noninvasive, as the surface of the skin is not breached, the insertion of such devices may cause discomfort for the patient. Further, the insertion and operation of such devices also involves the assistance of skilled medical personnel.
Carbon dioxide may also be measured transcutaneously by sensors held against a patient's skin. While these sensors are easier to use than respiratory gas sensors, they also have certain disadvantages. Because transcutaneous sensors depend upon the perfusion of carbon dioxide through a relatively thick epidermal layer, these sensors may not be as accurate. This problem may be addressed by measuring carbon dioxide that perfuses through a relatively thinner mucous membrane surface. However, a patient's mucous membrane is an aqueous environment, and surrounding water and other fluids may infiltrate a sensor, possibly damaging the sensing components and causing measurement inaccuracies.
Thus, it may be desirable to provide a water-resistant sensor for the measurement of carbon dioxide and other gases to protect a sensor that may be used in relatively aqueous environments, such as those containing mucous membranes.